1. Field of the Invention
The present invention relates to devices for the monitoring of flow resistances through the measurement of pressure differentials and, more particularly, to a device for the monitoring and visual indication of critical dirt accumulation in air intake filters.
2. Description of the Prior Art
It is known to measure the flow resistance across an air intake filter by means of a pressure gauge which, by indicating the pressure difference, or pressure drop, from the raw air space to the clean air space of the filter, gives a measure of the flow resistance encountered by the air. Since the flow resistance is a direct function of the accumulation of dirt on the filter element, an increase in the pressure drop across the filter element reflects a corresponding increase in the accumulation of dirt on the filter element.
The pressure in the raw air space of the filter can, as a rule, be equated with the atmospheric pressure. This makes it possible to use as a measure for the pressure drop the level of negative pressure inside the clean air space.
For the application under consideration, viz. the monitoring of dirt accumulation on the filter element of an air intake filter, the actual pressure level in the clean air space of the filter is not as important as the indication of changes in this pressure level over time. And, since the purpose of the device is essentially to give a warning signal, when the filter element requires cleaning or replacement, it may be sufficient for the device to merely give a reading of either "acceptable" or "unacceptable", using the colors green and red, for example.
A device of this type is known from the German Offenlegungsschrift No. 26 17 972. This publication discloses an indicator device which has a indicator slide arranged underneath a template with multiple window slots, so that a linear movement of the indicator slide brings red signal fields into alignment with the window slots, while shifting green signal fields out of the window slots. This linear movement of the indicator slide is indicative of the accumulation of dirt on the filter element of an air intake filter of an automotive internal combustion engine, for example.
The movement of the indicator slide is derived from a diaphragm piston which is arranged inside a housing and biased by a piston return spring in opposition to the action of the negative pressure on the underside of the piston. An axial displacement of the piston in response to a lowering of the pressure on the negative-pressure side of the piston is transmitted to the radially extending short arm of a bell crank of which the axially extending long arm drives the indicator slide along a radial, linear displacement path in a direction away from its rest position. The pivot axis of the bell crank is arranged at a lateral distance from and transversely to the axis of the diaphragm piston.
The indicator slide is biased in the direction towards its rest position by means of a slide return spring. The response of the indicator device can be calibrated through an adjustment of the compressive preload on the piston return spring.
A shortcoming of this known indicator device relates to the fact that the movement range of the indicator slide is very small and that, consequently, the reading of the indicator is not very accurate. Another shortcoming of this device is its restricted readability which is limited to a relatively narrow angle of vision.
It is also known form the prior art to translate the displacements of a diaphragm piston into rotary displacements of a pointer by using a bell crank which carries a gear segment on the extremity of its long arm. The gear segment drives a pinion on a shaft which carries the pointer. The axis of the pointer shaft is oriented parallel to the axis of the diaphragm piston.
Such a drive mechanism is suggested in the German Pat. No. 421 043. The mechanism uses a return spring which produces the opposing bias to the negative pressure on the diaphragm piston by engaging the long arm of the bell crank. This arrangement results in a higher bearing load on the bell crank. The off-center location of the pointer shaft represents a limitation to the movement range of the pointer.